Linux distribution used in security, "penetration testing" and by "ethical hackers".
"an open-source, Debian-based Linux distribution geared towards various information security tasks, such as Penetration Testing, Security Research, Computer Forensics and Reverse Engineering."
—
Named for the Hindu goddess of destruction and transcendental knowledge, the Kali Linux distribution is derived from Debian Linux, designed and built with a number of tools designed to simplify the task of investigating network and general computer security. The distro works just like any other; it allows one to use any of the many day-to-day tools (document preparation and the like), but also includes utilities for examining networks, and uncovering drive contents, including "sniffers" and password crackers. For this reason, it's seen as a Black Hat hacker distribution, and is especially portrayed in media as such. If you see a black-hoodied figure pounding away at a command line in, for example, Mr. Robot, Kali Linux is the distribution they're shown using.
Kali comes with all the tools you normally expect to be in a "normal" distro; office tools (word processor, spreadsheet and the like), but also a number of utilities gathered together to enable the user to poke at a system or network with the intent of uncovering its secrets. A pentester "Blue team" hacker has legitimate use for these tools, as they try to reveal security vulnerabilities. For the Black Hat hacker ("red team"), these tools are also valuable, but the intention is clearly very different; this is the media's representation of "hacker". A glance through installed and available software reveals so many tools with the word 'crack' embedded in them that it's clear the intent. Decryption tools, password crackers, network sniffing tools, scripts designed to ease the takeover of a system, all are either installed or available to the Kali user. A complete list may be found here.
Point of interest: despite being designed to use booted from a USB stick, many edgy folk still install it for daily use. This is generally frowned upon by the Linux installation issues. Whilst it has all the tools needed to be a daily driver, Kali is designed for penetration testing, and is unlikely to prove a secure, stable and long-term OS asset when installed. Equally, simply booting Kali Linux will not magically give you the ability to break into a system or crack passwords; there's a whole world of pre-existing knowledge and skills needed to do that.
With all that said, if you're still interested, bootable ISO images and a pile of documentation are available at htps://kali.org. Images are available for x86 and ARM for your Raspberry Pi cyberdeck build. There are containerised images and those designed for use in virtual machines or in the cloud. It's reportedly also available to install in the Windows Subsystem for Linux, if one is inclined to try it out.
Kali is not the only player in his space, other distributions with similar intent are available, notably Black Arch, a flavour of Gentoo Linux caked "Pentoo", Parrot Security OS, Samurai Web Testing Framework and Deft OS to name a few. It also has o be said that the individual tools are also available separately to install in most Linux distributions. Kali et al have just gathered them together and often given them a GUI or similar facelift for ease of use.
$ xclip -o | wc -w
544